Wave filter



y21,1940; G. HASSLR 2,201,338

WAVE FILTER Filed Dec. 3, 1957 INVENTOR GERf/ARD HASSL ER BY 1 (L ATTORNEY Patented May 21, 1940 I 2,201,338

WAVE FILTER Gerhard Hassler, Berlin-Friedrichshagen, Germany, assignor to General Electric Company, a corporation of New York Application December 3, 1937. Serial No. 177,867

In Germany December 9, 1936 3 Claims. (Cl. 178-44) In the communication field so-called wave filcircuit of Figure 3. In Figure 1 there is shown ters are employed for filtering individual freby way of example, a 1r type section of a band quency channels out of a frequency mixture. pass filter in which damping poles are produced These Wave filters have a passage range of frein the upper and lower blocking ranges at the quencies and one or several blocking ranges. In frequencies at which the fly wheel circuits in the 5' the blocking ranges the filters are to permit as series branch are anti-resonant. Now, if it is little passage of current as possible, i. e., they desired to locate these damping poles inthe proxmust have as high attenuation as possible. In imity of the limit frequencies which separate the order to attain this the wave filters are so dipassage range from the blocking ranges, coils of v mensioned, as is known, that in the blocking very low inductance must be employed. Since range or blocking ranges several damping poles the quality of the coils, i. e., the ratio between are present and distributed in a definite manthe reactance and the effective resistance canner. The word damping pole denotes a frenot be increased indefinitely, the antieresonant quency at which the attenuation is very high, impedances of the flywheel circuits cannot be fly theoretically speaking, at which the attenuation made sufiiciently high to produce very pronounced or damping becomes infinitely high. damping poles.

Such wave filters can be obtained in various In order to avoid this difficulty, it is proposed ways. They may be formed of bridge circuits in accordance with the invention, that the filter or equivalent circuits. In these circuits the sections which are to produce the damping poles damping poles are produced in that the bridge is lying in the proximity of the limit frequencies, be

perfectly balanced at these frequencies. In this formed as bridge members which are balanced way theoretically several damping poles can be only at the pole frequencies. The behaviour of produced in the blocking range or blocking these bridge type sections in the rest of the ranges. But in practice it was found that where blocking range need not -be considered. The several damping poles are required, it is almost finite resistances of the anti-resonance circuits an impossibility to balance the bridge so exof the bridge are readily balanced at the antiactly that a high damping be attained. In order resonant frequencies by means of suitably dito establish a perfect equilibrium of the bridge mensioned ohmic resistors in other parts of the it is in fact necessary that not only the reactive bridge.

components but also the resistive components be [Figure 2 shows the bridge member which is balanced. l the equivalent of the vr-member of Figure 1 It It is known that such wave filters may also be has the same wave resistance characteristic imbuilt up by the series connection of sections or pedance as the 1r-member. The energy is supchain parts of the typeknown as T- or w-circuit. plied to the terminals 1, I and derived at the In this type of circuit, the various damping terminals 2, 2. The two perfectly identical fly poles are arranged to occur in the different secwheel circuits S1 and S2 are connected to the tions. The damping poles are produced in that symmetrical differential transformer Us. If the at the definite frequency, the series resistance of filter consisted only of these-circuit elements, it the chain part which is to furnish the pole bewould at all frequencies be in a state of balance comes very high, or the parallel resistance very owing to the equality of the two fly wheel circuits, 40 low. This type of assembly has the advantage and would therefore possess a high blocking that it requires but a relatively small accuracy damping. But in order to have this high blockof the circuit elements. ing damping acting only at the damping poles of However, it was found, that owing to the unthe blocking ranges, there is placed in parallel to avoidable resistances of the reactances required thefly wheel circuit S2, the two fly wheel cirfor the assembly of the filter, the poles of the cuits S3 and S4 connected in series. At the antidamping curve have no longer a sufiiciently high resonant frequencies of these circuits the resistvalue especially when they lie in the proximity of ance thereof becomes very high, and the shunt to the limiting or cut-off frequencies of the filter. the fiy wheel circuit S2 will therefore be elim- In the drawing Figure 1 illustrates a 11' type inated at these frequencies. In order to bring section of a band passed filter according to the about at these frequencies a sufficiently perfect invention, Figure 2 is the equivalent bridge cirbalance of thebridge, there is placed in parallel cult of the filter of Figure 1, Figure 3 is another to the fiy wheel circuit S1 an ohmic resistor R form of the invention applied to a T type section which corresponds with the anti-resonance reof a filter, and Figure 4 is the equivalent bridge sistance of the fly wheel circuits S3 and S4. In

order that their resistance can be reproduced, these circuits must therefore have the same resistance at the anti-resonance frequencies. This equality can be brought about by connecting the resistor r in parallel to one of the two fly wheel circuits. If particularly high requirements are demanded of the filter, the reactance of the fly wheel circuit S4 is reproduced at the anti-resonance frequency of the fly wheelcircuit S3 by a fly Wheel circuit connected in series to the fly Wheel circuit S1. This fly wheel circuit is to be so dimensioned that at anti-resonance frequency of the fly wheel circuit S4, it reproduces the reactance of the flywheel circuit S3, and repro duces at the anti-resonance frequency of the fly wheel circuit S4, the wattless component of the fly wheel circuit S3. This oscillatory circuit is not represented in the drawing.

The invention however, not only relates to the above-described circuits, but may also be equally well applied to circuits with series resonance circuits. Figure 3 shows a T -section, in which series resonance circuits are inserted in the series branch and parallel branch. Figure 4 shows the equivalent bridge circuit. The energy is again supplied to the. terminals 5, I. The differential transformer Ue has the two series resonance circuits S1 and S2 connected thereto. S2 has in series therewith two further series resonance circuits S3 and S4 having at the pole frequencies a negligibly low ohmic resistance. This low resistance is reproduced at the otherside'of the bridge by the ohmic resistor R. In order to render the resonance resistance equal to the two series resonance circuits a corresponding resistor r can be connected in series to the one. If an extremely accurate balance is required then in amanner analogous to the above-described example, the reactance of the series resonance circuit S3 or S4 which is not in resonance must likewise bereproduced on the other side of the bridge. At the terminals 2 and 2 the energy is derived from the filter.

What I claim is:

1. A transmission network adapted to be connected in series with a band pass filter having a pair of cut-off frequencies and large attenuation at frequencies remote from the range of frequencies between said cut-01f frequencies but relatively little attenuation at frequencies closely adjacent said cut-off frequencies, said network being in the form of a Wheatstone bridge having arms so constituted as to produce a substantial reactance balance at two rejection frequencies,

one of which is closely adjacent one of said outoff frequencies and the other of which is closely adjacent the other of said cuteoif frequencies, and additional resistances in said bridge so arranged as to produce a resistance balance of said bridge at said two rejection frequencies.

2. A transmission network adapted to be connected in series with a band pass filter having a pair of cut-off frequencies and large attenuation at frequencies remote from the range of frequencies between said cut-off frequencies but relatively little attenuation at frequencies closely adjacent said cut-off frequencies said network being in the form of a Wheatstone bridge having a parallel resonant circuit in one arm and a similar circuit shunted by a pair of series-connected parallel circuits resonant to the rejection frequencies forming a second arm of the bridge, the bridge arms being so constituted as to produce a substantial reactance balance at the two rejection frequencies, one of which is closely adjacent one of said cut-off frequencies and the other of which is closely adjacent the other of said cut-olf frequencies, a resistance in shunt to the circuit in the first arm and a second resistance in shunt to one of the circuits in the second arm, said resistances producing a resistance balance of said bridge at said two rejection frequencies.

3. A transmission network adapted to be connected in series with a band pass filter having a pair of cut-off frequencies and large attenuation at frequencies remote from the range of frequencies between said cut-off frequencies but relatively little attenuation at frequencies closely adjacent said cut-01f frequencies, said network being in the form of a Wheatstone bridge having a series resonant circuit in one arm and a similar circuit having serially connected thereto a pair of parallel connected series resonant circuits resonant to the rejection frequencies forming the conjugate arm of the bridge, the bridge arms being so constituted as to produce a substantial reactance balance at the two rejection frequencies, one of which is closely adjacent one of said cut-off frequencies and the other of which is closely adjacent the other of said outoff frequencies, a resistance in series with the circuit in the first arm and a second resistance in series with one of parallel connected series circuits in the second arm, said resistances producing a resistance balance of said bridge at said two rejection frequencies.

GERHLARD HASSLER. 

